For marine propellers, propeller blade pitch is often defined in terms of "inches", i.e., defining the distance that a boat would be propelled through the water by a single revolution of the propeller, assuming no slippage, e.g., a propeller having a pitch of "13 inches", is one having the blade angle necessary to linearly advance the boat 13 inches upon one complete revolution of the propeller.
It has similarly been well understood that the conditions under which the boat will operate are important in determining the optimum pitch for the propeller, for an engine producing a certain maximum power output. Such operating conditions include the load, intended speed, and the type of hull, of the boat being propelled. For example, when a boat was to be used for towing a water-skier, i.e., a relatively heavy load, a propeller having a lower pitch would be selected, e.g., approximately a 15" pitch for a relatively small, 16 feet long outdoor pleasure boat with a 100 h.p. engine. Similarly, a higher speed boat with, e.g. a 300 h.p. engine, would use a relatively high pitch blade, e.g. a 21-inch pitch propeller.
Past workers have designed propellers which have manually resettable blade pitch positions. The pitch was set before starting the engine, and the pitch remained constant during continued engine operation. Such a device is shown for example in U.S. Letters Pat. No. 3,790,304. Other past designs have manually resettable blade positions that allow changes in the blade pitch position during operation. These have provided for manual adjustments made via mechanical, hydraulic or electric means. Such devices are shown for example in U.S. Letters Pat. No. 2,554,716; 3,216,507; and 4,599,043.
The prior art, recognizing the utility of propellers which vary blade pitch during operation of the engine, have devised various means of changing the pitch either in accordance with a self-actuating design, i.e. the pitch automatically changes based upon changes in operating conditions, e.g., engine RPM, or by operator-controlled means, such as pneumatic or hydraulic controllers. Self-actuating propellers, which are apparently continuously variable over a range of pitch positions, are suggested for marine propellers by Reid in U.S. Pat. No. 3,177,948, and for aircraft propellers by Lagrevol and Biermann, in U.S. Pat. Nos. 2,669,311 and 2,694,459. A propeller, especially adapted for an outboard engine for marine use, having both manual and automatic self-actuating variable pitch means, is shown in U.S. Pat. No. 2,682,926, to Evans.
Other devices which provide for automatic, self-actuated changes in blade pitch positions, wherein the blades are spring biased against change, is shown for example in U.S. Pat. Nos. 2,290,666, 2,988,156; 3,145,780; 3,204,702; 3,229,772; 3,231,023; 3,295,610; and 3,567,336. In addition, there have been variable pitch marine propeller designs which are actuated by a sudden, or sharp, change in engine RPM to provide the necessary impetus to shift the blade pitch. Examples of such devices are shown in U.S. Pat. Nos. 3,275,083 and 3,302,725.
Prior self-actuating propellers intended primarily for uses on aircraft have incorporated means to lock the blades in one or more blade positions. Such devices are shown for example in U.S. Letters Pat. Nos. 2,669,311 and 2,694,459, and German Patent publication No. DE 3,429,297.